1. Field of the Invention
The present invention relates to an inspection method and an inspection system for inspecting a line width or a positional of a pattern formed on an object to be inspected, such as a mask.
2. Background Art
Recently, the line width of circuits required for semiconductor devices has become increasingly narrower due to further integration and increases in capacity of large-scale integrated circuits (LSI). Semiconductor devices are manufactured via circuit formation by exposure transfer of a pattern on a wafer with a reduced projection exposure device called a “stepper”, using an original image pattern with a circuit pattern formed thereon, these are called masks or reticles (hereinafter referred to collectively as masks). Masks used to transfer such fine circuit patterns to the wafer are manufactured by electron beam writing apparatuses, which can write micropatterns. Further, the development of a laser beam lithography device, which can write using a laser beam is also being attempted. An electron beam lithography device is also used when directly writing a circuit pattern onto a wafer.
Since the cost of manufacturing LSIs is very high, an increase in yield is required to make the manufacturing economically feasible. Meanwhile, in recent representative logic devices, formation of a pattern with a line width of several-ten nanometers (nm) is now required. Pattern defects in the mask and fluctuations in the process conditions during exposure transfer may be large factors in the reduction of yield. Further, major factors that reduce the yield include a mask containing a pattern defect and a variation in conditions of the exposure transfer. As a result, in the mask inspection process, dimensions which must now be detected as pattern defects are miniaturized, and it becomes necessary to detect extremely small positional errors of the pattern. Thus, high accuracy is required for inspection systems to inspect the dimensions of masks used in LSI manufacture.
As a method for detecting pattern defects, mention may be made of a die-to-die inspection system and a die to database inspection system. The die-to-die inspection system is used when the mask to be inspected has thereon a plurality of identical chip patterns, or a plurality of chip patterns each including an identical pattern segment. According to this system, since mask patterns are directly compared, a highly accurate inspection can be carried out with a relatively simple device configuration. However, defects, which are common to both patterns being compared cannot be detected. On the other hand, in the die-to-database inspection method, an actual pattern on a mask is compared to reference data generated from the design pattern data that was used to manufacture the mask. Thus, this method allows exact comparison of the pattern with the design pattern data, although the required system size is large since the method requires a processing system for generating a reference image. There is no choice but to use this inspection method when the mask to be inspected has only one chip pattern to be transferred to the wafer.
In die-to-die inspection, light is emitted from a light source, and the mask to be inspected is irradiated with this light through an optical system. The mask is mounted on a table, and this table is moved so that the emitted beam of light scans the surface of the mask. Light transmitted through, or reflected from the mask, reaches an image sensor via a lens, thereby forming an image thereon. The optical image thus formed on the image sensor is sent to a comparing unit as measurement data. The comparing unit compares the measurement data with reference data in accordance with an appropriate algorithm, and if they are not identical, the mask is determined to have a defect (See Japanese Patent Application Laid-Open (JP-A) No. 2008-112178)
In a conventional inspection system, a mask pattern image, obtained by imaging an optical image with an optical image sensor is determined to be correct. However, with the recent miniaturization of a device pattern on a mask, it is difficult to distinguish the difference between an unwanted pattern defect and the correct pattern. Furthermore, if positional defects or a line width difference of the pattern are measured using this method, a problem arises in that the measured value fluctuates depending on the pattern. This means that the positional defects or the line width difference across the entire surface of the mask cannot be accurately calculated. This kind of problem is prominent in a logic mask which has many patterns, and thus the development of a method which can measure accurately each pattern is necessary. The need to accurately control the dimensions of patterns has thus increased the difficulty of manufacturing masks. As a result, there has been a loss in the yield of masks that meet the required specifications, thereby raising the cost of mask manufacturing.
The present invention was created in consideration of the above circumstances, and an object of the present invention is to provide an inspection method and an inspection system that can accurately calculate the distribution of line width difference or the distribution of positional aberration of a pattern formed on a mask and a reference pattern.
Other challenges and advantages of the present invention are apparent from the following description.